A well-developed theoretical literature — dating back at least to Nordhaus (1969) — has analyzed optimal patent policy design. We re-present the core trade-off of the Nordhaus model and highlight an empirical question which emerges from the Nordhaus framework as a key input into optimal patent policy design: namely, what is the elasticity of R&D investment with respect to the patent term? We then review the — surprisingly small — body of empirical evidence that has been developed on this question over the nearly half century since the publication of Nordhaus's book.